Grating light valves are known as devices that can selectively diffract an incident beam of light. A variety of known grating light valves are discussed in the prior art and some others are or were commercially available. One grating light valve is described in U.S. Pat. No. 5,311,360. A similar grating light valve and a method of making it are described in two U.S. Patent applications, Ser. No. 08/482,188 entitled: FLAT DIFFRACTION GRATING LIGHT VALVE, now U.S. Pat. No. 5,841,579, issued Nov. 24, 1998, and Ser. No. 08/480,459 entitled: A METHOD OF MAKING AND AN APPARATUS FOR A FLAT DIFFRACTION GRATING LIGHT VALVE, now U.S. Pat. No. 5,661,592, issued Aug. 26, 1997, both filed on Jun. 7, 1995. These two patent applications are not admitted as prior art. Each of these three patent documents is incorporated herein by reference. The discussion that follows is in no way intended to modify or alter the scope of the teachings or claims of any of the above three captioned references. Rather, this discussion is intended only to schematically describe these references insofar as it will aid in understanding by providing bases for comparing or contrasting those technologies to the present invention.
According to the teachings of these three references, a diffraction grating light valve is formed of substantially parallel ribbon structures. The ribbons are formed over a semiconductor substrate using conventional semiconductor processing steps such as those used for forming integrated circuits. FIG. 1 shows the preferred grating light valve 10 from the U.S. Pat. No. 5,311,360. Each of the ribbons 18 have an upper surface coated with a reflective material 20, such as aluminum. In the spaces between the ribbons, the substrate 16 is also coated with the reflective material 24. The height difference between the reflective material 20 on the ribbons 18 and the reflective material 24 on the surface of the substrate 16 is 1/2 the wavelength .lambda. of an expected beam of light. Because of this height difference, the beam of light reflects from the surface of the grating light valve essentially as if it were a specular mirror as shown in FIG. 2.
Upon applying a predetermined voltage potential across the ribbons 18 and the substrate 16, the ribbons 18 are caused to deflect downwards and contact the substrate 16. The grating light valve 10 is constructed so that the height difference in this deflected state is 1/4 the wavelength .lambda. of the expected beam of light. Because of this height difference, the beam of light is diffracted at the surface of the grating light valve essentially as shown in FIG. 3.
FIG. 4 shows a cross section view of two adjacent ribbons according to the technology taught in the two above captioned patent applications in an undeflected and reflecting state. According to the applications, in an undeflected state all the ribbons are in an up position. All the reflecting surfaces are on ribbons rather than having alternate ones of the reflectors mounted on the substrate as in U.S. Pat. No. 5,311,360. The ribbons are selectively deformable by coupling the ribbons to external control circuitry. When the ribbons for a single grating light valve are all in an up position, an essentially flat specular mirror is presented to an incident beam of light. The mirror is necessarily broken by the gaps between the ribbons of a single grating light valve structure.
FIG. 5 shows a cross section view of two adjacent ribbons according to another technology in a deflected and diffracting state. Alternate ones of the ribbons within a single grating light valve are selectively deformed and deflected into contact with the underlying substrate. When this occurs, the grating light valve diffracts the incident beam of light.
For both of the technologies described above, a voltage is coupled to the selected ribbon and to the substrate (or an appropriate conductor mounted on the substrate) for effecting the deflection of one or more ribbons. Though FIG. 6 is a graph schematically illustrating intensity versus voltage for both technologies described above, the graphs formed from empirical measurements from these two technologies are not identical. It will be understood that the light from the incident beam is collected from the diffracting angle away from the incident beam of light. Thus, no light is collected and accordingly there is no or low intensity when the ribbons are not deflected and thus the grating light valve is acting as a specular mirror. When the ribbons are deflected, the incident beam of light is diffracted to the collection point and the collected intensity is large.
Because the ribbons snap between completely up and completely down states, the intensity of the collected light is either fully "on" or fully "off". If a ribbon could be moved a partial distance between up and down, the light would diffract through another angle and the intensity of collected light could be varied accordingly. Unfortunately, partially moving a ribbon with the technologies taught by these three references is impractical as described below.
As is readily apparent from FIG. 6, there is hysteresis in the ribbons. As the voltage applied across the ribbons and substrate is increased, the intensity essentially does not change until a first threshhold voltage V.sub.D is reached. Then, the intensity increases very dramatically for a very small increase in voltage as the ribbons snap down to the substrate. The true empirical graphs are not vertical in this region but are very, very steep. Similarly, once the ribbons have snapped down, the intensity essentially does not change until a second threshhold voltage V.sub.U is reached. Then, the intensity decreases very dramatically for a very small decrease in voltage as the ribbons snap back to its up state. Again, the true empirical graphs are not vertical in this region but are very, very steep. The second threshhold voltage V.sub.U is lower than the first threshhold voltage V.sub.D.
Theoretically, it is possible to move a ribbon only partway between an up and relaxed position to a down and deflected position. However, because of the very small changes in voltage that are required to account for such a change, it is difficult to control or effect such partial deflections. The problem is compounded because the electrical characteristics of a ribbon changes as it deflects. These electrical changes are due at least in part because the mechanical strain induced by deflecting the ribbons changes the electrical impedance of the ribbons. Also the capacitance of the ribbon-substrate system changes as the distance between the ribbons and the substrate changes. Additionally, because the ribbons are selectively deformable and because a plurality of ribbons are electrically coupled together for addressing, the capacitance changes are impossible to predict except on an imprecise statistical basis.
It would be desirable to provide a system that allows precise control of ribbon movement between fully up and fully down.
In an unrelated conventional display technology an electron gun is used to illuminate pixels, such as in commercial television and computer monitor displays. Such displays have been used for many years and the properties for controlling the electron beam are well known. However, because the illumination is developed by the electrons impinging on a display structure, the intensity of the display image is related to the beam power. Accordingly, systems of this type draw considerable power to develop a suitably intense display image. Reducing power consumption is generally considered a common goal of all system designers for a variety of reasons, including commercial, economic and environmental. It would be desirable to provide a display system that reduces the power consumption of the display without deteriorating the quality of the display.
Another unresolved problem with electron gun displays is a health related issue. It is as yet undetermined by the medical community whether a continual bombardment by electrons causes any health issues. The potential health problems due to the physical configuration of the display tube. The electron gun is mounted within a tube and is disposed away from the viewing surface (screen). The gun shoots electrons at the screen. As the electrons strike the structure of the screen, photons are excited and emitted for viewing. Some portion of the electrons will necessarily pass through the screen and travel beyond striking whatever object lies in its path. A person working each day at a computer monitor will be continually bombarded with electrons. It would be desirable to provide a display system that does not bombard a viewer with electrons.